SPR1-897 Always refresh indexers in _set_keep
SPR1-897 Always refresh indexers in _set_keep

The main route into `_set_keep` is via `DataSet.select`, which always
passes actual arrays as parameters. All other routes also involve at
least one stage 1 selection, so the indexers will always be refreshed.
The `update_all` and `update_flags` checks are therefore not only
confusing, but also unnecessary, so cull them.

ska-sa/katdal